The present invention relates to electro-optical cells. More specifically, it is concerned with an electro-optical cell for liquid crystal display of (electric) field effect twist nematic type (hereinafter referred to as FETN type) which is excellent in visibility angle, contrast, response, reliability and other characteristics.
A cell for liquid crystal display of FETN type can be basically obtained by interposing a nematic liquid crystal compound or composition having a positive dielectric anisotropy (when simply referred to as liquid crystal material hereinafter, it is intended to include both the compound and composition) between a pair of oriented transparent electrode base plates each comprising a transparent base plate and an electrode film provided thereon, and sealing the peripheral parts of the plates by any suitable means.
In a cell for liquid crystal display of FETN type, molecules of the liquid crystal material are initially aligned parallel to the electrode base plate by the influence of a pair of electrode base plates whose surfaces have been orientated thereby to impart optical rotatory power to the liquid crystal material. This alignment is controlled or realigned by the application of a suitable electric field thereby to vary the optical rotation through the liquid crystal material. By this variation in the optical rotation, electric signals are converted into optical information. By utilizing such mechanism, the FETN type cell can be used in displaying apparatuses.
Because of this operational principle, uniformity of the initial alignment of the liquid crystal (i.e. the alignment when no electric field is applied) or uniformity of the (surface) orientation of the electrode base plates giving the uniformity of the initial alignment of the liquid crystal is of primary importance in order that the cell for liquid crystal display of FETN type will function satisfactorily. Secondly, the seal structure of the cell must have excellent tight-sealing property and waterproofness in order to maintain the above-mentioned alignment and to prevent the liquid crystal material from deteriorating. Generally, a liquid crystal material is hydrolyzable or subject to some deterioration under the action of moisture even if it is not hydrolyzed, and at the same time the orientation itself is necessarily influenced by moisture.
Schiff-cyano base liquid crystal materials which have been widely used in recent years as a liquid crystal material having a positive dielectric anisotropy suitable for use in the cell for liquid crystal display of FETN type are particularly highly hydrolyzable. When the hydrolysis proceeds, the consumption of electric power is increased, and at the same time the nematic-isotropic transition temperature (NI point) of the liquid crystal material is decreased until it exhibits no liquid crystal phase. There are also other types of liquid crystal materials such as biphenyl liquid crystal materials which are not hydrolyzable and exhibit little decrease in the NI point even if they are subjected to high temperatures. When these materials are attacked by moisture, the consumption of electric power also increases, however, and the alignment of the liquid crystal material is consequently destroyed.
Therefore, in order to obtain an excellent electro-optical cell for liquid crystal display of FETN type, both the uniformity of the initial alignment of the liquid crystal and the soundness of the seal structure of the cell are essential factors. The present invention contemplates improving these two factors and providing an electro-optical cell for liquid crystal display of FETN type which has excellent optical characteristics such as visibility angle, contrast, response, and reliability.
As a measure known heretofore for obtaining uniformity of the initial alignment of a liquid crystal (i.e. a measure for orientating electrode base plates), there is a method wherein the electrode base plates are rubbed in one direction (orientated) by a material such as cloth, but by this method, resulting alignments of liquid crystal molecules differ locally, and the uniformity of orientation is not sufficient. Furthermore, the orientation is lost within a short time.
According to another method intended to solve the above described problems, a surface-active agent (surfactant) of a certain kind is additionally used, and the electrode base plates are rubbed in one direction (as disclosed, for example in IBM Technical Disclosure Bulletin Vol. 13, No. 11, Apr. 1971). While the uniformity of orientation is improved to a certain extent by this method, the surfactant does not possess sufficient heat resistance and, furthermore, gives rise to deterioration of the liquid crystal material. Then, when an electric field is applied, the surfactant decomposes because of the electric field and undergoes deterioration, and the orientation is destroyed.
On the other hand, there are known two types of seal structure; one type using an organic sealing agent as a sealing agent, and the other type using an inorganic sealing agent. Of these, an organic sealing agent is deficient in waterproofness and airtightness and cannot provide an electro-optical cell having high reliability as stated above. In this respect, an inorganic sealing agent is preferable. However, the inorganic sealing agent widely used in the electronic components field is generally a low melting point glass frit whose melting and processing temperature is above 350.degree. C. Therefore, where such a sealing agent is used for sealing the cell, orientation capable of withstanding such a high temperature is necessary, and it is further necessary to protect the electrode film without damaging the seal properties of the sealed parts. Although some glass frits may be fused at a temperature below 350.degree. C, they are inferior in bonding strength, and the portions bonded thereby may be destroyed by heat impact.
Accordingly, in order to provide an electro-optical cell for liquid crystal display with high reliability which has excellent tightness, bonding strength, and heat impact resistance, orientation treatment which results in an orientation resistant to a temperature of at least 350.degree. C, and preferably no less than 400.degree. C is required. The orientation treatment comprising rubbing the electrode base plate simply or using a certain type of surfactant as referred to before cannot fully produce a heat resistant orientation.
At the present stage, as an orientation method which impacts a high heat resistance, i.e. a reliable orientation such that the inorganic sealing agent can be used, an inclined evaporation orientation method known (as disclosed, for example, in Applied Physics Letter, 173 21. 4, 1972). According to the inclined evaporation orientation method wherein the electrode base plates are inclined relative to an evaporation deposition source of an inorganic material such as silicon monoxide, and the material is deposited by evaporation thereby to accomplish orientation, the orientation agent itself is certainly heat resistant. However, the resulting orientation is relatively poor in heat resistance and the orientation power is low so that the molecules of the liquid crystal are not completely aligned horizontally to the electrode base plate. For this reason, a liquid crystal apparatus using the cell so obtained is inferior in contrast and visibility angle.
On the other hand, the orientation is gradually varied when an electric field is applied for a prolonged period of time, and even when no electric field is applied, parts of the surface provided with the electrode film and the remaining parts of the surface provided with no electrode film of the electrode base plate differ in orientation direction from each other.
That is, in the present state of the art, an electro-optical cell for liquid crystal display of FETN type having excellent uniformity of the initial alignment of the liquid crystal in combination with high reliability of seal structure has not yet been realized in practice.
In order to overcome the above described various difficulties accompanying the known electro-optical cells of FETN type and to provide an electro-optical cell of FETN type possessing both excellent display performance and high reliability, we have carried out studies with the realization that the construction of the cell and the selection of material must be collectively examined in relation to the production method including the sealing method. As a result, we have attained the present invention.